Detection of trace explosives is of great concern for the homeland security, battlefield protection, and industrial and environmental safety control. Fluorescence quenching based sensing has proven to be one of the most promising approaches for trace explosives detection, for which various conjugated polymers, molecular imprinted polymers, dye-doped silica and metal-organic frameworks have been fabricated into films. Fluorescence quenching of these materials upon exposure to vapors of nitroaromatic explosives (e.g., TNT, DNT) has been explored. However, the quenching efficiency of these materials is often limited by short exciton diffusion due to the poor molecular organization and/or weak intermolecular electronic interactions as usually observed for polymer based materials. As a result, reliable results for very low concentrations can be difficult to achieve and sensitivity can be limited.
Recently, an alternative approach to increase fluorescence sensing efficiency by fabricating rigid aromatic molecules into nanofibrils has been reported. These novel one-dimensional nanostructures possess long range exciton diffusion due to the extended intermolecular π-π electronic interaction. Upon deposition onto a substrate, the nanofibrils form a nanoporous film in a variable range of porosity through entangled piling of the individual fibrils. The large surface area to volume ratio and porosity thus formed, in combination with the amplified fluorescence quenching relied on the enlarged exciton diffusion, usually enable expedient, effective vapor detection of nitroaromatic explosives. Particularly, for the building-block molecules containing carbazole as the conjugation unit, the nanofibrils thus fabricated demonstrate not only high sensitivity in vapor sensing, but also strong selectivity towards nitrobased explosives against other common chemical liquids and solids. However, the macrocyclic conjugation structure of these molecules imposes significant difficulty during synthesis because synthesis of these macrocyclic demands special catalysts for making the precursors. Consequently, these materials are currently limited in practical application due at least in part to these difficulties. To make the nanofibrils more practical in vapor sensing, the inventors determined to find alternative building-block molecules that are easier to synthesize but still maintain the chemical properties and features suited for fabrication into nanofibrils and fluorescence sensing of nitrobased explosives.